i think its because of entropy.
In lattice enthalpy the gaseous ions go from:
gaseous + gaseous ---> solid.
In hydration the ions go from
gaseous + gaseous ---> gaseous + aqueous.
Im both cases its decreased in entropy. meaning that when we use this equation:
ΔG=ΔH-TΔS
TΔS>ΔH so it wont dissolve